In long distance transmission systems at high bit rates over optical fibers, signal distorting effects occur which reduce the eye opening at the receiver. The distortion may be due to linear or non-linear effects, such as group velocity dispersion (GVD), polarization mode dispersion (PMD), transmitter chirp, extinction ratio, or intersymbol interference (ISI) induced by non-linearities of the fiber. These effects reduce the eye opening at the receiver and thereby lead to a reduced tolerable optical signal to noise ratio (OSNR), which is crucial for optically amplified systems.
In the optical receiver, the optical power is converted back into electrical signals. The digital data and sampling clock phase has to be derived from the analog signal by means of a clock and data recovery circuitry (CDR).
In order to improve the signal quality at the CDR circuit, it is known to apply adaptive equalization. A well known adaptive equalizer is the Feed Forward Equalizer (FFE) for a compensation of the transmission channel. In a FFE, the signal is equalized by means of a weighted sum of a tapped delay line.
The adaptation of the tap weights requires information about the analog signal. The information is utilized to estimate the channel model and/or for the calculation of the equalizer parameters.
It is known in the art to use a full speed analog to digital conversion (ADC) or a sample and hold circuit with an additional ADC, which is working at a lower speed.
An alternative method for a determination of the tap weights is based on measurement of pseudo errors at additional variable thresholds. The control algorithm has to vary and adapt equalization parameters by means of a deterministic procedure, e.g. by a least mean square algorithm (LMS) or by dithering and evaluation of the direction of improvement.
The major problem for the generation of a control signal is the realization of a high speed sample and hold circuit to sample the analog signal or the implementation of an “eye monitor”, which may be understood as additional threshold with an exclusive-or-function of the current threshold. The output pulses counted at the exclusive-or-gate (EXOR-gate) correspond to the number of the different decisions. Those parallel comparator structures always exhibit performance degradation since the generation of an analog signal is always distorted by various parasitic elements of the circuitry. Using a pseudo error counter instead is disadvantageous since the data rate is doubled which results in an increased power consumption of the output interface.
The data stream transmitted may include a forward error correction code (FEC) which improves the bit error rate (BER) for a given signal-to-noise ratio (SNR) by reducing stochastic distortions from optical or electrical noise and cross talk. For high bit rate transmission, FEC is becoming more and more mature to increase the tolerable SNR on long haul transmissions. In encoding for forward error correction, redundant bits are added to a bit stream so that errors may be detected and corrected at the far end. The number of added bits may equal the number of signal bits, resulting in a doubling of the data transmission rate for a given channel. However, in many cases redundant transmission by using FEC is beneficial due to a guaranteed low error rate.
Different FEC-Coding-Schemes are used, such as so called in band or out band, BCH (Bose-Chaudhury-Hoequengheen) or RS (Reed-Solomon) codes which fit for Sonet/SDH digital wrapper formats. If the input error rate of the data stream is below the error correction capability of the respective error correction code, the bit errors can be corrected and estimates of a bit error ratio (BER) may be measured by using the additional information from the respective FEC-decoding scheme. Specifically, the number of errors that can be corrected amount to (d−1)/2, where d denotes the minimum number of bit positions by which code words for a particular code are different (“Hamming distance”).